This invention relates to a method of making a high purity substituted diphenyldisulfide by reacting a substituted thiophenol with hydrogen peroxide in the presence of a catalytic amount of a basic salt. In particular, this invention relates to performing the reaction in a two phase water-organic solvent system where the symmetrical product is less soluble in the organic solvent than the unsymmetrical product, and therefore the purity of the product is greater than the purity of the thiophenol starting material.
High purity 4,4'-dichlorodiphenyldisulfide is used to make various pharmaceuticals such as bisphosphonates. It can be made by oxidizing parachlorothiophenol with an oxidizing agent, such as bromine, but this route is expensive and results in the production of large amounts of waste hydrogen bromide. Hydrogen peroxide can also be used to make this product from parachlorothiophenol, but, because the starting material also includes some orthochlorothiophenol, the product contains both the symmetrical 4,4'-dichlorodiphenyldisulfide isomer and the unsymmetrical 2,4'-dichlorodiphenyldisulfide. The presence of small quantities of the 2,4'-dichlorodiphenylsulfide in the product is not acceptable for the production of pharmaceuticals.